1. Field of the Invention
The present invention relates to an on-line calibration system for calibrating chemical monitors which analyze a fluid sample, and more particularly, to a calibration system which conditions a portion of the fluid sample to be analyzed by the chemical monitor to a predetermined chemical characteristic so that the chemical monitor is calibrated with respect to the predetermined chemical characteristic of the conditioned portion of the fluid sample.
2. Description of the Related Art
The control of impurities in and the chemical characteristics of power plant steam cycle water is recognized as an important and necessary measure in protecting a power plant against corrosion related failures and insuring plant reliability. Various on-line monitors are available for detecting chemicals and chemical characteristics, including dissolved oxygen, pH, sodium, hydrazine, ammonia, specific conductivity, and cation conductivity, in a flowing fluid sample, e.g., a stream of steam cycle water. For accurate control of the chemical characteristics of steam cycle water, on-line monitors must be calibrated on a regular basis.
In order to accurately calibrate a chemical monitor, it is necessary to eliminate or compensate for the effects of any ionic, or chemical, species in the fluid sample, other than the chemical characteristic being monitored, which vary the response of the monitor but which do not alter the particular chemical characteristic being monitored. Further, the sensors and electrodes used in conjunction with many monitors respond differently in a stagnant solution than in a flowing stream. Thus, accurate calibration requires that the sensor be placed in a flowing stream during calibration. In addition, it is desirable to have an in situ calibration procedure which can be automated since monitors are located in a variety of different locations in a power plant and since it is mandatory to confirm that the information generated by on-line monitors is accurate and reliable whenever a steam process disturbance is detected.
Calibration of chemical monitors, for example, pH monitors, historically has been accomplished with an off-line procedure using concentrated buffer solutions. In calibrating a pH monitor, a pH electrode is placed in the buffer solution and a pH meter is set to the known pH of the buffer solution. Then, the electrode is returned to a flow cell to detect the pH of a flowing sample. Re-calibration requires the manual removal of the electrode from the flow cell to place the pH electrode in the buffer solution. However, such an off-line calibration procedure is difficult to automate and fails to eliminate the effects of ionic species in the fluid sample which do not affect chemical characteristics being monitored, but which do have an effect on the response of the sensor or electrode used to perform the monitoring, thereby creating an erroneous pH measurement when the pH electrode is used to analyze the fluid sample. Off-line calibration using a buffer solution is often inaccurate for three further reasons: first, flowing fluid samples influence the response of a sensor or electrode differently than stagnant solutions; second, many sensors and electrodes tend to respond differently in buffer solutions than they do in high-purity fluid samples having minimal buffering capacity; and third, temperature differences between the buffer solution and the fluid sample are not accounted for in the calibration procedure.
Various systems have been proposed to calibrate chemical monitors. For example, a pH monitor having an automatic buffer standardization system for automatically, periodically standardizing the monitor against a buffer solution of a known pH is disclosed in U.S. Pat. No. 4,151,255-Capuano et al. In this system, the pH measuring means has pH electrodes which are located in a test chamber. During a pH measurement, sample fluid is fed to the test chamber. To calibrate the pH measuring means, the sample fluid is drained from the test chamber, the test chamber is rinsed with a rinse fluid, and then a buffer solution is introduced into the test chamber. Thus, the pH measuring means can be adjusted with respect to the known pH of the buffer solution. This system, however, does not eliminate or compensate for ionic species in the sample fluid which have an effect on the response of the pH electrode, nor does it address the problem of variations in the response of a pH electrode to a concentrated buffer solution versus a high purity fluid sample having minimal buffering capacity. Further, during a measurement, the sample fluid fed to the test chamber accumulates therein and exits only through an overflow opening. Thus, the pH electrode responds to the pH of the entire volume of sample fluid collected in the test chamber over a period of time, rather than the pH of the sample fluid provided to the test chamber at any one point in time. Accordingly, the pH measurement provided by this system is a profile of the pH of the fluid sample provided to the test chamber over a period of time and instantaneous or continuous on-line measurements of the pH of a particular portion of the sample fluid are not possible.
Another system for calibrating a sensing electrode is disclosed in U.S. Pat. No. 4,490,236-Petty. This apparatus provides for the measurement of a sample solution with an electrode and for the calibration of the electrode using a test solution. The system includes a cell body having a cavity for receiving a sensing electrode, a reservoir for holding a calibration solution, and a valve provided in the cell body for controlling the flow of the calibration solution to the cavity. The valve is held in a position which prevents flow of the calibration fluid to the cavity by the force of the flowing sample fluid. When the flow of the sample solution is terminated, the valve automatically opens to provide a flow of calibrating solution through the cavity. Thus, calibration is performed using a highly buffered calibrating solution; as a result, this apparatus does not address the problem of the varied response of a pH electrode in highly buffered solutions versus the response of pH electrode in high purity steam cycle water having a minimal buffering capacity.
A variety of dissolved oxygen monitors for continuous on-line operation are available. The most common method for calibrating such dissolved oxygen monitors, most of which are based on a membrane covered polarographic sensor, involves the exposure of the sensor to gas mixtures of known oxygen concentration, including a zero oxygen gas, to establish the monitor zero. Another calibration method involves the measurement of dissolved oxygen in a series of standards and a comparison with a standard, recognized analytical procedure such as the Winkler method. Other suggested methods for calibrating dissolved oxygen monitors include adding oxygen to the fluid sample by electrolytic generation of oxygen. The electrolytic generation of oxygen is reasonable for on-line calibration; however, the addition of an electrolytic cell to a dissolved oxygen monitor significantly increases the cost and complexity of the monitor. Other methods, such as catalase decomposition of hydrogen peroxide, are only useful for off-line calibration and involve elaborate procedures which are difficult to automate or routinely implement in power plants. Thus, most dissolved oxygen monitors in power plants are calibrated at a single point corresponding to 20% oxygen concentration, by exposing the sensor to atmospheric air, and are assumed to have a linear response based on a zero point determined by the zero current output of the polarographic sensor. However, the dissolved oxygen measurements for power plant steam cycle water in most power plants are in the 0-20 ppb range--a measuring range which differs from the calibration point by four orders of magnitude. Therefore, a high degree of accuracy of the monitor in the measuring range cannot be expected, and readings taken with different monitors calibrated by this method are often quite different.